1. Technological Field
The disclosed technology relates generally to image sensors and more specifically to active pixel sensor imaging systems.
2. Description of the Related Technology
Conventional four-transistor active pixel sensors (APS) have configurations in which a source-follower drives the signal onto a readout column line that is shared amongst all the pixel sensors in that column. In the case of large imagers with large pixel size and/or a large number of pixels, parasitics associated with readout column lines may become considerable, with capacitances up to 10 pF, slowing high-speed image sensor readout as the settling time of column line signal becomes a bottleneck. A solution to increase the readout speed is increasing the biasing current of the column line this approach increases power consumption and increases the voltage drop over the large column line resistance, thus decreasing voltage headroom.
U.S. Pat. No. 7,847,846 B1 discloses an active pixel sensor that employs readout transistor current sensing and feedback circuits to increase current driving capabilities and readout speed. The current through the in-pixel readout transistor is sensed by a circuit that is external to the pixel, and according to the measured current value a feedback current is supplied to charge the read-line parasitic capacitance. The feedback current is supplied by a circuit that also is external to the pixel area.
There is a need to improve current state of the art active pixel sensor imaging systems in order to reduce power consumption and/or settling time and/or a trade-off thereof.